j.g. pausas' blog

Cork oak (Quercus suber) typically grows in relatively wet mediterranean environments [1]. However there are some cork oaks in arid climate; perhaps the population in the driest site is the small and isolated cork oak patch in Rambla de Talón (ca. 100 m asl, Ribera de Molina, Molina de Segura, Murcia, Spain; Fig. 1). It includes less than 100 individuals scattered in an area of sandy conglomerates (Fig. 3); the average rainfall is less than 300 mm. They are believed to have been planted in the past (when?), but their persistence in such arid conditions gives them a high added value. This population is much smaller and is located in a much drier conditions than the one in Pinet (Valencia) we mentioned some time ago [2,3].

Precipitation during the last spring was above average, and currently (end of October 2020) most oaks in Rambla de Talón look healthy and have some acorns. Of the 26 tree we look at, the number of acorns ranged from 0 (7 trees) to more than 400 acorns (2 trees), but most trees have less than 10 acorns (Fig. 2; median= 5 acorns). In addition, there is no evidence of recruitment from previous years. That is, if persistence of this population is desired, it would require some help for their regeneration. Given that they produce some acorns, restoration actions using local acorns is possible.

References
[1] Aronson J, Pereira JS, Pausas JG (eds). 2009. Cork Oak Woodlands on the Edge: conservation, adaptive management, and restoration. Island Press, Washington DC. 315 pp. [The book]

[2] Pausas JG, Ribeiro E, Dias SG, Pons J & Beseler C. 2006. Regeneration of a marginal Cork oak (Quercus suber) forest in the eastern Iberian Peninsula. J. Veget. Sci. 17: 729-738. [pdf | doi | wiley ]

[3] El surar de Pinet – a small isolated population of cork oak. jgpausas.blogs.uv.es/2018/10/15/

One of the fire adaptations in some trees is a thick bark that protects stem buds and growing tissues from the high temperature of fire [1,2]. Cork oak (Quercus suber) is an outstanding example of a tree with this fire adaptation; it a Mediterranean tree that has a very thick insulating bark (the cork) that enables the tree to survive even high intensity fires and to resprout epicormically after fire [3-5]. The great characteristics of the cork, a natural, versatile and sustainable product, has made the cork a raw material for many uses. The cork is extracted from the trees every 9 to 12 years, and regrowth after that. The industrial characteristics of cork are many, including thermal and acoustic insulator, odorless, very light, elastic and compressible, with low capillarity, no toxic, imputrescible when dry, impermeable to liquid and gases, resistant to damage, non-flammable, organic, anti-static, hypoallergenic, and with natural touch. Consequently cork has been used for a wide range of products, although the most well-known cork product are the bottle stoppers. But the best is that the tree survives after cork extraction, and in fact, the use of cork justifies the conservation of private cork oak forests. The short message is: drink wines with cork stopper!

Fig. 1. The cork products that I have at home.

 Fig. 2. Other products made from cork. Photos taken in: Tunisia (A,C,D), the cork museum of Palafrugell, Girona, Spain (B, I), a shop in Tempio, Sardinia, Italy (E), the cork museum of Aggius, Sardinia, Italy (F,G,H).

References

[1] Pausas, J.G. 2015. Bark thickness and fire regime. Funct. Ecol. 29:317-327. [doi | pdf | suppl.]

[2] Pausas J.G. 2017. Bark thickness and fire regime: another twist. New Phytol. 213: 13-15. [doi | wiley | pdf] 

[3] Aronson J., Pereira J.S., Pausas J.G. (eds). 2009. Cork Oak Woodlands on the Edge: conservation, adaptive management, and restoration. Island Press, Washington DC. [The book]  

[4] Pausas, J.G. 1997. Resprouting of Quercus suber in NE Spain after fire. J. Veg. Sci. 8: 703-706. [doi | pdf]

[5] Pausas J.G. & Keeley J.E. 2017. Epicormic resprouting in fire-prone ecosystems. Trends Plant Sci. 22: 1008-1015. [doi | sciencedirect | pdf]  

More on: cork oak | bark and fire |

Cork oak (Quercus suber) is a strong fire-resistant tree species thank to is very thick and insulating corky bark [1-4]. In fact it is the only European tree with the capacity to resprout from epicormic buds in the canopy after an intense crown-fire [1]. However, the bark of the cork oak is periodically harvested for cork production (mainly for bottle tops but also for other uses, [2]) and thus bark harvesting increases the vulnerability of the tree to fire. In a recent paper we quantified the response of cork oak (tree mortality, stem mortality, and crown recovery) after fire [5]. The results showed that fire vulnerability was higher for trees with thin bark (young or recently debarked individuals) and decreased with increasing bark thickness until cork was 3–4 cm thick. This bark thickness corresponds to the moment when exploited trees are debarked again, meaning that exploited trees are vulnerable to fire during a long period. Exploited trees were also more likely to be top-killed than never-debarked trees, even for the same bark thickness. Additionally, vulnerability to fire increased with burn severity and with tree diameter, and was higher in trees burned in early summer or located in drier south-facing aspects. All these aspects need to be considered when managing cork oak woodlands specially nowadays that fire activity is increased [6]. Increasing the length of the cork harvesting cycle would increase the time during which the trees have a thicker bark and are better protected against fire injury. Since cork is the main economical income from these forests, stopping bark exploitation might be unrealistic in most cases. However, in fire-prone areas where conservation and tourism are the main objectives, stopping bark explotation would likely be the most effective option to increase ecosystem resilience to fire. The valorisation of many other services provided by cork oak forests [7] could create economic incentives to decrease the bark-exploitation dependency of these systems in the future.

Foto: Cork oak  resprouting from epicormic buds (By F. Catry)

References

[1] Pausas, J.G. 1997. Resprouting of Quercus suber in NE Spain after fire. J. Veg. Sci. 8: 703-706. [doi | pdf]

[2] Aronson, J., J. S. Pereira, and J. G. Pausas (eds). 2009. Cork Oak Woodlands on the Edge: Ecology, Adaptive Management, and Restoration. Island Press, Washington, DC. [web of the book]

[3] Pausas J.G. 2009. Convergent evolution. jgpausas.blogs.uv.es, 8/Nov/2009. [link]

[4] Pausas J.G. 2011. Bark thickness: a world record? jgpausas.blogs.uv.es, 3/Jan/201. [link]

[5] Catry F., Moreira F., Pausas J.G., Fernandes P.M., Rego F., Cardillo E. & Curt T. 2012. Cork Oak vulnerability to fire: the role of bark harvesting, tree characteristics and abiotic factors. PLoS ONE 7: e39810. [doi | pdf ]

[6] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]

[7] Bugalho M.N., Caldeira M.C., Pereira J.S., Aronson J., & Pausas J.G. 2011. Mediterranean Cork oak savannas require human use to sustain biodiversity and ecosystem services. Frontiers in Ecology and the Environment 9: 278-286. [doi | pdf | blog]

Cork oak (Quercus suber, from the western Mediterranean Basin[1]) is a weird oak. In most oak species, acorn maturation pattern is clear and fixed. In some species acorns mature in one year, in others acorn require two years for maturation. This trait is not fixed in Cork oak, some trees have annual acorns, some others have mainly biennial acorns, and some trees have both. This is why when we relate Cork acorn production with climatic variables the relation is very weak (explained variance < 8%), much weaker than for other oaks. However, after the trees being grouped according to their dominant acorn maturation pattern (annual or biennial), weather parameters account for 44% of the variability in acorn crops, with trees with annual acorns exhibiting mast fruiting in years with reduced spring frost and shorter summer droughts and trees with biennial acorns showing the opposite pattern [2]. Thus, conditions that negatively affect annual production could be beneficial for biennial production (and vice versa). The ability to modulate the acorn production pattern of a given year according to the environmental conditions could be regarded as an example of phenotypic plasticity for facing variable and uncertain climatic conditions, such as those in Mediterranean ecosystems. To what extent other oaks living under variable and stressful conditions behave similarly remains to be explored.

Figure: Recently debarked Cork oak and cork oak landscape in eastern Spain (foto: J. Cortina)

[1] Aronson, J., J. S. Pereira, and J. G. Pausas (eds). 2009. Cork Oak Woodlands on the Edge: Ecology, Adaptive Management, and Restoration. Island Press, Washington, DC. [web]

[2] Pons, J. and J. G. Pausas. 2012. The coexistence of acorns with different maturation patterns explains acorn production variability in Cork oak. Oecologia [doi | pdf]

Other post on Cork oak:

• Conservation of cork oak ecosystems, Mar 14th, 2011 [link]
• Bark thickness: a world record?, Jan 3rd, 2011 [link]
• Wine supporting biodiversity, Jan 5th, 2010 [link]
• Cork Oak Woodlands on the Edge, Oct 14th, 2009 [link]

Mediterranean cork oak (Quercus suber) savannas, which are found only in southwestern Europe and northwestern Africa, are ecosystems of high socioeconomic and conservation value. Characterized by sparse tree cover and a diversity of understory vegetation, these ecosystems require active management and use by humans to ensure their continued existence. The most important product of these savannas is cork, a non-timber forest product that is periodically harvested without requiring tree felling. Market devaluation of, and lower demand for, cork are causing a decline in management, or even abandonment of cork oak savannas. Subsequent shrub encroachment into the savanna’s grassland components reduces biodiversity and degrades the services provided by these ecosystems. In contrast, poverty-driven overuse is degrading cork oak savannas in northwestern Africa. “Payment for ecosystem services” schemes, such as Forest Stewardship Council (FSC) certification or Reducing Emissions from Deforestation and Degradation and enhancement of carbon stocks (REDD+) programs, could produce novel economic incentives to promote sustainable use and conservation of Mediterranean cork oak savanna ecosystems in both Europe and Africa.

Bugalho M.N., Caldeira M.C., Pereira J.S., Aronson J., & Pausas J.G. 2011. Human-shaped Cork oak savannas require human use to sustain biodiversity and ecosystem services. Frontiers in Ecology and the Environment 9: 278-286 [doi | pdf] [featured on the cover: pdf, foto]  podcast

Aronson J., Pereira J.S., Pausas J.G. (eds). 2009. Cork Oak Woodlands on the Edge: conservation, adaptive management, and restoration. Island Press, Washington DC. 315 pp. [the book]

More posts on oaks.

Foto: D. Crespo (Portugal)

The thickness of the bark is a trait of paramount importance in trees living in ecosystems with frequent surface (understory) fires (e.g., some coniferous forests, savanna woodlands, etc.). This is because the bark is a good insulator protecting vital tissues from the heat of the fire. Having a bark few millimeter thicker provide an advantage in such fire-prone ecosystems. Thus there has been a selection for thick barks in surface fire ecosystems [1]. A prominent example of a tree with a very thick and insulating bark is the Cork oak (Quercus suber) that growth in the western part of the Mediterranean Basin [2]. In such species the thicker is the bark, the better is the response after fire [3, 4]. This bark is so thick and insulating that it is used not only as bottle tops, but also as insulating material in many industrial applications. However the Mediterranean Basin has been densely populated from long ago and it is very difficult (if possible) to find Cork oak woodlands in “natural” conditions, and thus it is not easy to know how thick the bark of Cork oak could attain in natural conditions. Most trees are frequently debarked for obtaining cork (frequencies ranging from every 9 to every 12 years, depending of the site conditions).

Few days ago I visited an ethnographic museum in Aggius (Sardinia) and found a piece of Cork oak bark of about 22 cm thick (see picture below), which is pretty thick. I only know of one record of a thicker bark: 27 cm in a 140 years-old Cork oak that was never debarked [5]. Do you know of any tree (of the same or another species) in the world with a thicker bark? Is Cork oak the world record on bark thickness?

Figure: Piece of bark from a Cork oak (Quercus suber), in the ethnographic museum of Aggius (Sardinia).

References:

[1] Pausas J.G. 2009. Convergent evolution. jgpausas.blogs.uv.es, 8/Nov/2009. [link]

[2] Aronson J., Pereira J.S., Pausas J.G. (eds). 2009. Cork Oak Woodlands on the Edge: conservation, adaptive management, and restoration. Island Press, Washington DC.  [link]

[3] Pausas, J.G. 1997. Resprouting of Quercus suber in NE Spain after fire. J. Veg. Sci. 8: 703-706. [doi | pdf]

[4] Catry F.X., Rego F., Moreira F., Fernandes F.M., Pausas J.G. 2010. Post-fire tree mortality in mixed forests of central Portugal. Forest Ecology & Management 206: 1184-1192. [doi | pdf]

[5] Natividade J.V. 1950. Subericultura. Direçao Geral dos Serviços Florestais e Aquícolas Lisbon, Portugal.

In September 2003, a mixed forest of central Portugal (Tapada Nacional de Mafra) burned in a large crown fire. We surveyed the survival of more than 700 trees during 4 years postfire. The results are detailed in a recent paper by Catry et al. (2010, [1]) ; the table below provides a summary on the proportion of the trees that  (a) died, (b) survived but were top-killed (stem and crown mortality) and resprouted from the base, and (c) the stem survived, after 4 years postfire (for tree mortality, the value observed after the first year is given in brackets).

Most pines (P. pinaster and P. pinea) died and few, specially of P. pinea, were little affected by fire; there were a significant positive relationship between crown damage and tree mortality.
Most broadleaved trees survived the fire, and whether the stem survived or died (and resprouted from the base) were related to bark thickness and char height (i.e. fire severity). Castanea sativa showed the highest tree mortality, mostly due to post-resprouting mortality after the first year. Fraxinus angustifolia, Olea europaea and Pistacea lentiscus showed no mortality at all; most Olea and Pistacea individuals resprout from the base, while for Fraxinus the crown of most trees were unaffected. The low effect of fire in Fraxinus angustifolia is probably due to the topographic positions where this species occurs; in addition theses trees are quite tall and with relatively thick bark. Quercus coccifera and Q. faginea showed low mortality and most trees resprouted from the base. Quercus suber showed almost no mortality and almost all trees showed epicormic resprouting, due to their extremely thick and insulating bark [2, 3].

References

[1] Catry F.X., Rego F., Moreira F., Fernandes F.M., Pausas J.G. 2010. Post-fire tree mortality in mixed forests of central Portugal. For. Ecol. Manage. 206: 1184-1192. [doi] [pdf]

[2] Pausas, J.G. 1997. Resprouting of Quercus suber in NE Spain after fire. J. Veg. Sci. 8: 703-706. [pdf]

[3] Aronson J., Pereira J.S., Pausas J.G. (eds). 2009. Cork Oak Woodlands on the Edge: conservation, adaptive management, and restoration. Island Press, Washington DC. 315 pp. [The book] [Ch 1, the tree]

[Update] see: “To resprout ot not to resprout”, Jan 25th, 2012.

Good news: Sainsbury’s to pop new corks for wildlife. All of Sainsbury’s own-brand wines will be sealed with corks certified by the Forest Stewardship Council by the end of 2010 [see The Guardian, 31/Dec/2009].  Sainsbury is the third largest chain of supermarkets in the United Kingdom. We hope other supermarkets and wine makers will follow this initiative.

Remeber that IUCN proposed ten things we all can do to save biodiversity [see], and one was to only drink wines with natural cork stoppers!

Cork oak (Quercus suber) is a WWF priority species, because it is one of the most ecologically, economically and/or culturally important species.

For more information on cork oak woodlands see the book Cork Oak Woodlands on the Edge, and the WWF Cork Oak Programme.

Images from two different tree species (A and B), from different Families (and different Orders), taken in different continents…

A1 A2 A3

B1 B2

The thick bark offers protection to fire and thus these species are both adapted to live in fire-prone ecosystems [1].

Can you guess the species name of A and B?    [ Answer: A | B ]

Notes

[1] See also: The ecology of bark thickness | The ecology of bark thickness (2): another twist

Coca M. & Pausas J.G. 2009. Regeneration traits are structuring phylogenetic diversity in cork oak (Quercus suber) woodlands. J. Veget. Sci. 20: 1009-1015  [Wiley] [doi] [pdf]

• Question: What factors determine the deviations from the relationship between species richness (which considers species as independent entities) and phylogenetic diversity (PD) (which considers species relatedness)? What are the implications for community composition and phylogenetic structure?
• Location: Los Alcornocales Natural Park, in southern Iberian Peninsula (Spain).
• Methods: We recorded all woody species and geographical features on 94 (20 m × 20 m) plots of cork oak woodlands. Disturbance information was obtained from the Park records; precipitation was estimated from local maps. PD was computed as the minimum total length of all the phylogenetic branches spanning the set of species on each site. Then, PD was regressed against species richness to test to what extent the unexplained variance in this relationship could be accounted for by environmental variables and disturbances, and against the representation of species with different regeneration strategies.
• Results: Species richness and PD are strongly related; however, the remaining variability can be explained by: (1) precipitation and disturbance, and (2) the proportion of seeder species. Thus, the PD both of areas with low precipitation and high disturbance, and of areas with a high representation of seeder species, is lower than what would be expected from the species richness.
• Conclusions: Regeneration traits are important in structuring plant community composition; specifically, they contribute to shaping biodiversity in Mediterranean ecosystems. Species richness tends to overestimate biodiversity in highly disturbed systems.

The relationship between the residuals from the phylodiversity-species richness regression, and the proportion of post-disturbance seeding species (P+; r= -0.560, p< 0.0001). Negative residuals indicate lower phylogenetic diversity than expected from species richness values, that is, a tendency for phylogenetic clustering.

IUCN propose ten things we all can do to save biodiversity [see], and one is to only drink wines with natural cork stoppers! Cork stoppers do not pollute the environment (as opposite to plastic stoppers), but also their use save the Cork oak woodlands. These forests face a major threat: the growing use of plastic and metal substitutes for cork stoppers in wine bottles, cork’s main market. If the economic value of cork oak forests is not maintained cork oaks will be cleared for other land uses. Cork oak (Quercus suber) is a WWF priority species, because it is one of the most ecologically, economically and/or culturally important species.

For more information on cork oak woodlands see the book Cork Oak Woodlands on the Edge, and the WWF Cork Oak Programme.